Our aim is to investigate the stereochemistry of thiol-disulfide interchanges in order to understand better their role in biological processes, especially radiation protection and carcinogenesis. We plan to achieve our objective by studying the crystal structures of thiols, sulfides, mixed sulfides and disulfides, and thiol-disulfide and thiolsulfide complexes using x-ray diffraction techniques. By analyzing the short non-bonded atomic contacts to sulfur in these structures in terms of the chemical and electronic nature of the sulfur and contacting atoms, we will develop a stereochemical picture of the reaction path or paths. Using this picture of the thiol-disulfide interchange reaction, we will develop and evaluate proposals for the role of this important reaction in biological processes such as radiation protection, carcinogenesis and protein renaturation. We shall study chemical agents that protect against radiation by thiol-disulfide interchanges and develop criteria which will be of some help for designing anti-radiation agents.